1. Technical Field
The present invention relates to a polymer electrolyte fuel cell, and in particular, relates to a membrane electrode assembly for use in a polymer electrolyte fuel cell.
2. Background Art
Recently, exhaustion of petroleum resources is a crucial issue, and environmental problems such as air pollution and global warming caused by consumption of fossil fuels have become serious. Under these circumstances, fuel cells have attracted much attention as a clean power source for electric motors in which carbon dioxide is not generated, and such fuel cells are being widely developed and used.
In the case in which such a fuel cell is used in a vehicle, a polymer electrolyte fuel cell in which a polymer electrolyte membrane is used is desirably used since high voltage and large current can be obtained. A membrane electrode assembly for the polymer electrolyte fuel cell is produced as follows: a catalyst such as platinum is carried by a catalyst carrier such as carbon black; a pair of electrode catalytic layers is made by unifying the catalyst and an ion conducting polymer binder; a polymer electrolyte membrane having ion conductivity is disposed between the electrode catalytic layers; and a gas-diffusion layer is formed on each of the electrode catalytic layers. Furthermore, a separator which also functions as a gas passage is formed on each of the gas-diffusion layers to obtain a polymer electrolyte fuel cell.
In such a polymer electrolyte fuel cell, a reducing gas, such as hydrogen or methanol, is introduced at one electrode catalytic layer (fuel electrode) through the gas-diffusion layer of the fuel electrode side, and an oxidizing gas such as air or oxygen is introduced at the other electrode catalytic layer (oxygen electrode) through the gas-diffusion layer of the oxygen electrode side. In the fuel electrode, due to the existence of the catalyst in the electrode catalytic layer, protons (H+) and electrons are generated from the reducing gas, and protons migrate to the electrode catalytic layer of the oxygen electrode side through the polymer electrolyte membrane. In the oxygen electrode, due to the existence of the catalyst in the oxygen electrode, protons react with the oxidizing gas introduced at the oxygen electrode and electrons to produce water. Therefore, by electrically connecting the fuel electrode and the oxygen electrode with a lead, a circuit in which electrons generated in the fuel electrode migrate to the oxygen electrode is formed, and electric current is obtained.
Conventionally, in the above-mentioned membrane electrode assembly, perfluoroalkylenesulfonic acid polymer compound (e.g., Nafion (Trademark), produced by DuPont) is used as a polymer electrolyte membrane. The perfluoroalkylenesulfonic acid polymer compound has both sufficient proton conductivity and chemical resistance as a fluorine resin since the compound is sulfonated.
In Japanese Unexamined Patent Application Publication No. 2001-192531, as an alternative ion conductive material for a perfluoroalkylenesulfonic acid polymer, for example, an aromatic hydrocarbon based polymer compound is disclosed to produce the membrane electrode assembly for a polymer electrolyte fuel cell.
In such a polymer electrolyte membrane, hydrogen ions are diffused through water clusters formed at hydrophilic channels (ion conductive channels). Therefore, the amount of water absorbed and fixed in the ion conductive component of the membrane and channel formation formed by the ion conductive component of the membrane are thought to be important.
However, in the membrane electrode assembly having the above-mentioned polymer electrolyte membrane, ion conductivity may be greatly deteriorated because of drying due to low humidity and water freezing at low temperatures.